Insomnia is a prevalent public health problem affecting large segments of the population on an occasional, recurrent, or chronic basis. Persistent insomnia is associated with significant impairments in daytime functioning, reduced quality of life, and increased health-care costs. Despite evidence that cognitive-behavior therapy (CBT) is an effective and well accepted treatment for insomnia, a significant proportion of individuals do not respond adequately to this treatment modality. Hence, there is a need to identify the active therapy components and mechanisms of change in order to develop more effective therapeutic approaches and optimize outcomes. The specific aims of the proposed study are to (a) evaluate the effects of behavioral versus cognitive therapies for nighttime sleep disturbance and associated daytime impairment, (b) investigate the mechanisms of change and, (c) examine the collateral impact of insomnia therapies on psychological symptoms and psychiatric conditions commonly comorbid with insomnia (selected anxiety disorders and depression). A sample of 186 adults with chronic insomnia will be recruited from two sites (Laval University and University of California, Berkeley). Participants will be randomly assigned to one of three groups: (a) behavior therapy (BT;n = 62), (b) cognitive therapy (CT;n = 62), or (c) cognitive-behavior therapy (CBT;n = 62). Measures of outcome (sleep/insomnia, daytime functioning) will be administered at baseline, end of treatment, and at 6- and 12-month follow up. Measures of mechanisms of change (maladaptive sleep habits and schedules, dysfunctional beliefs and attitudes, sleep-related worry, monitoring, thought suppression) will be administered at baseline, after the 4th and 8th therapy sessions, and at the end of treatment. It is expected that (1) BT and CBT will be more effective for improving sleep, relative to CT and (2) CT and CBT will be more effective for reducing daytime functional impairment, relative to BT. It is also expected that mechanisms of change will be specific to each therapeutic approach. Finally, as CT targets maintaining mechanisms shared across various psychiatric disorders, whereas BT targets mechanisms specific to insomnia, it is hypothesized that CT will be more effective than BT in reducing comorbid psychological symptoms and psychiatric disorders. The public health significance of the proposed study is that it will provide useful information to improve our understanding of insomnia and to enhance efficacy and efficiency of therapeutic approaches for a prevalent and costly health problem. The long-term objective is to contribute to the development and dissemination of evidence-based treatments for chronic insomnia and its common comorbidities.